In Society of Automotive Engineers (SAE) paper 930650, "A Complete Frontal Crash Sensor System - I", by Breed, Sanders, and Castelli, which is included herein by reference, the authors conclude that an airbag crash sensor system should include an electro-mechanical arming sensor having a long dwell. Many vehicle airbag sensor systems use a sensor mounted forward in the crush zone of the vehicle. However, there are crashes where the crush zone sensor can close, indicating that a crash is in progress which requires the deployment of the airbag, when the arming sensor, which is normally mounted back in the passenger compartment, has closed but reopened due to the dynamics of the crash. This is known as insufficient dwell and is characteristic of most arming sensor designs.
Insufficient dwell can have several causes depending on the particular sensor design. In one case it can result when the sensing mass does not have sufficient over-travel, that is additional travel after the contacts are closed. In one case, for example, a one mile per hour velocity change is required to close the sensor and an additional two miles per hour is all that is required for the sensing mass of the sensor to reach its maximum travel. If during the crash there is a reversal in the crash pulse and the passenger compartment, where the arming sensor is typically mounted, speeds up by more than 2 miles per hour, the arming sensor will open and may stay open for an extended period of time. An example of this situation would a multiple accident on a foggy highway. If car number 2, for example, strikes the rear of car number 1 and the front of car number 2 experiences a 8 MPH velocity change when car number 2 is in turn struck in the rear by car number 3, the front of car number 2 will continue to slow down, perhaps by an additional 8 MPH causing the crush zone sensor to trigger, while the passenger compartment may be increasing its velocity causing the arming sensor to open. By the time that the arming sensor closes again, the crush zone sensor may have become disconnected and the airbag will not deploy.
Insufficient dwell can also be caused by the effects of cross-axis vibrations. These vibrations are those which occur in the vertical and lateral directions. Arming sensors are designed to sense longitudinal crash pulses. Sensors with sliding sensing masses have been shown to be strongly affected in both the time to trigger and the dwell when strong cross-axis vibrations are present at a level which occur in many types of crashes. Sensors based on a ball in a tube geometry have also been shown to be sensitive to cross axis vibrations which can also increase the time to fire and decrease the dwell.
In vehicles having both driver and passenger airbags, it is sometimes required to stage the deployment of the two airbags so as to reduce the noise and the pressure in the vehicle which would otherwise occur if both airbags deployed simultaneously. In these cases, the passenger airbag is sometimes delayed as much as 20 to 30 milliseconds from the time that the driver airbag deployment is initiated. This delay increases the possibility that the arming sensor will open either due to a reversal in the velocity or to the normal return of the sensing mass at the end of the crash signal. This latter occurrence is most likely either in marginal crashes or in short duration crashes. Most arming sensors have an increasing bias with sensing mass travel. The bias is the force on the sensing mass which returns it to its initial position where the contacts open at the end of the crash. Many arming sensors are biased with a spring and the force on the mass increases as the mass moves away from its initial position compressing the spring. When the crash pulse is over, therefore, the mass can be rapidly propelled back to its initial position.
Although most arming sensors have a relatively short dwell, one exception is a sensor based on a band and roller design. This sensor has a long dwell but is large and relatively expensive.
Another example of the prior art is a sensor which uses a sliding magnet as a sensing mass where the magnet passes close to a reed switch causing the reed switch contacts to attract each other and close after a particular displacement. The main advantage of this sensor is that it is small and inexpensive and, in one configuration using a dual sensing mass, a moderate dwell is achieved. One problem, however, is that this sensor has a single set of contacts with a limited current carrying capacity. If the current associated with an airbag deployment passes through the reed contacts they weld closed. Another problem is that it is very sensitive to cross-axis vibrations which can prevent the sensor from triggering or cause it to trigger late with reduced dwell on some airbag required crashes.
Dual sets of contacts in the arming sensor are desired by some system designers in order to isolate the reserve energy supply for the passenger airbag from the one for the driver airbag. In some cases, the first airbag which triggers can become shorted for a period sufficient to drain the energy from the reserve supply and prevent the second airbag from deploying. For this reason, dual energy reserves are sometimes used which then requires dual sets of contacts in the arming sensor.
It is also desirable for the contacts to open after the airbag has deployed. Unlike the crush zone sensor which usually exhibits significant visual damage after a crash, the arming sensor is mounted in the passenger compartment where its visual appearance is unaltered after the crash. A principle of good sensor design is that if the sensor is not visibly damaged, it must be functional. Since the contacts can weld closed on certain reed switch sensors, the sensor may appear good from the outside but have welded contacts inside. In this case the sensor might be inadvertently reused in a vehicle which is repaired after a crash where the airbags deployed. There is an especially strong motivation to reuse the airbag when it is part of an expensive electronic sensor and diagnostic package.
The invention disclosed and illustrated below is intended to solve the problems and limitations of the prior art discussed above.